Use of photo catalytic material for self-cleaning banknotes

ABSTRACT

Self-cleaning banknotes are provided using coatings, inks and additives which are photo-active and catalytic to reactions which are effective in breaking up organic contaminants or dirt to allow for the self-cleaning of banknotes by ambient light exposure as well as the cleaning of processed banknotes using equipment with more intense optical excitation, thus increasing their usable life. The invention is usable with all substrates and particularly polymeric substrates such as biaxially-oriented polypropylene (BOPP). The invention further discloses a system which allows a certain class of fitness parameters to cause these banknotes to be redirected to a cleaning module, be reevaluated, and then either returned to circulation or rejected and/or destroyed. In addition, inks which are photo-catalytic can be used for extending the life of the banknotes in printed regions.

BACKGROUND OF THE INVENTION

The present invention relates generally to substrate materials thatinclude a self-cleaning feature. More specifically, the presentinvention relates to a substrate that includes a photo-catalyticmaterial therein that, upon exposure to a source of illumination, reactsin a manner that causes the paper, or in general, the substrate materialto shed accumulated contaminants and dirt. The process is also effectivein disinfection of the banknotes.

High security documents such as banknotes are generally formed onsubstrate materials that are frequently equipped with security elementsthat are difficult to imitate and permit even a layman to check theauthenticity of the print or the document. Such security elements can befor example windowed security threads which are visible in certain areason the surface of the paper of value, applied foils which have atransparent or metallized embossed hologram, blind embossings, so-called“latent images” produced by printing technology or by printing andembossing technology which render different information from differentviewing angles, prints containing optically variable pigments andproducing different color effects depending on the viewing angles, orprints comprising metallic effect ink which have metallic luster forexample in a gold, silver or bronze tone. In addition to these unaidedfeatures, there are quasi-public security threads, fibers and inks whichfluoresce or phosphoresce under illumination with UV or IR sources.

As counterfeiters have become more sophisticated, the security featuresin such documents have had to become more advanced as well in order toprevent widespread fraud. As such substrates have become more advanced,their cost to produce has also gotten much higher and therefore makingthe replacement of worn currency quite expensive. Therefore it isimportant that in addition to being secure, such substrates must have ahigh level of durability.

Banknotes have a finite time in circulation due to soling and tearing ofthe notes in use by the public. Banknotes are handled in many waysduring their usable life and experience a variety of mechanical stressesas well as coming into contact with substances that can dirty the notes,making them hard to authenticate and use. One of the major determinantsof the banknote life, which is shortest for the lowest denominations, issoiling. Work by the Dutch National Bank has shown that the primarysource of soiling is sebum from contact with fingers and whicheventually oxidizes and becomes yellow. As a result, durability must bebalanced with the other important components of value documents, such asbanknotes, is their flat substrate which typically consistspredominantly of cotton and cotton-denim mixtures and pulp papers whosetypical haptics is also influenced by the one-sided or two-sidedcalendaring during steel engraving. The haptic character of a bank noteis manifested mainly by its feel and its flexural stiffness; itfurthermore has a characteristic sound when being deformed and creased.

In order to improve durability of these substrates, it is known toprovide papers of value with a dirt-repellent protective layer to extendthe life time and fitness for circulation. Such a protective layertypically contains cellulose ester or cellulose ether for the greaterpart and micronized wax for a lesser part and which is applied to thebank notes all over. The micronized wax is dispersed by kneading ormixing with oil, an ink binder or a mixture thereof. The sheets freshlyprinted with the protective layer can be stacked without difficultieswithout any black ink from one sheet staining the sheet therebelow

Another coating composition containing only a binder and no fillers isapplied to the banknote paper, which has a large surface area or highsurface roughness due to its porosity. The composition is applied in alayer thickness such that a smooth surface and thus little possibilityfor dirt deposit results. Further, the coating is thin enough not toimpair the other stated properties of the paper.

The problem is that known protective layers do not last or wear well.Conventional protective layers comprising water-based lacquers usuallyfail to completely meet a demanding requirement profile. For example,very good dirt repellence and adhesion quality go against resistance tothe penetration of liquid, and vice versa. Water-based lacquerstherefore currently meet the high requirements for a protective layer insecurity printing and in particular bank-note printing only if a secondcomponent in the form of a crosslinking agent is added.

The problem is that central banks need to replace worn and soiled notesat a cost to taxpayers. In the USA, typically, the volume of notesmanufactured is in the billions of notes per year (4-6 billiontypically). The production of banknotes is costly and particularly sofor the higher denominations which have many security features, both forthe public as well as machine readable by bill acceptors and the centralbanks using high speed sorters. Banknote sorters made by Geiseke andDevrient, De La Rue International and Toshiba typically processbanknotes at rates of 10-40 banknotes/second and perform a number ofdiagnostics using sensors in the note path. Theses sensors are acombination of authentication sensors as well as note fitness sensors.The fitness sensors primarily use imaging and analysis of the images todetermine if the banknote should be destroyed or returned intocirculation.

The cost of replacing banknotes has led to the development of polymericsubstrates such as biaxially oriented polypropylene (BOPP) as well ascomposite substrates of polymer and paper. These substrates have begunto replace paper banknotes as they claim to have more durability andsoil resistance and hence save the central bank money by replacing fewerunfit banknotes every year.

There is a need therefore for a substrate material that provides highsecurity against fraud while exhibiting increased durability andself-cleaning characteristics. Further there is a need for a substratethat through the use of coatings, inks and additives that arephoto-active and catalytic to reactions which are effective in breakingup organic contaminants or dirt. There is still a further need for asystem that using a certain class of fitness parameters causesidentified banknotes to be redirected to a cleaning module prior tore-evaluation and then a determination that they are either returned tocirculation or destroyed.

BRIEF SUMMARY OF THE INVENTION

In this regard, the present invention provides for substrate materialsthat include a self-cleaning and directed cleaning capability. Morespecifically, the present invention provides a substrate that includes aphoto-catalytic material therein that, upon exposure to a source ofillumination, reacts in a manner that causes the substrate and printinginks materials, to shed accumulated contaminants and dirt and a systemfor sorting, cleaning and analyzing banknotes formed using such asubstrate material.

In general the present invention employs coatings, inks and additiveswhich are photo-active and catalytic to reactions which are effective inbreaking up organic contaminants or dirt. In some cases,photo-excitation of these materials also leads to interfaces which aresuper-hydrophilic allowing for more water to be split, producing morereactive OH radicals by the photo-catalytic materials and in turnbreaking more organic contaminant bonds for eventual removal of theadhered materials (oxidized sebum and other contaminants). Such aneffect desirably occurs through exposure to ambient light. Theself-cleaning materials incorporated in banknotes can be additionallyutilized beyond ambient light exposure using equipment with optical/UVexcitation and flowing air, water flow, and mechanical brushes, thusremoving soiling and increasing the usable life of banknotes.

The invention is usable with all substrates and particularly polymericsubstrates such as biaxially-oriented polypropylene (BOPP). Theinvention further discloses a system which allows a certain class offitness parameters to cause these banknotes to be redirected to acleaning module, be reevaluated, and then either returned to circulationor destroyed. In addition, inks which are photo-catalytic can be usedfor extending the life of the banknotes in printed regions.

It is therefore an object of the present invention to provide asubstrate material that provides high security against fraud whileexhibiting increased durability and self-cleaning characteristics. It isa further object of the present invention to provide a substrate thatthrough the use of coatings, inks and additives that are photo-activeand catalytic to reactions which are effective in breaking up organiccontaminants or dirt can be altered by light to allow for theself-cleaning of banknotes by ambient light exposure as well as thecleaning of processed banknotes using equipment with optical excitation,thus increasing their usable life. It is still a further object of thepresent invention to provide a system that using a certain class offitness parameters causes identified banknotes to be redirected to acleaning module prior to reevaluation and then a determination that theyare either returned to circulation or destroyed.

These together with other objects of the invention, along with variousfeatures of novelty which characterize the invention, are pointed outwith particularity in the claims annexed hereto and forming a part ofthis disclosure. For a better understanding of the invention, itsoperating advantages and the specific objects attained by its uses,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated a preferred embodiment of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings which illustrate the best mode presently contemplatedfor carrying out the present invention:

FIG. 1 is a diagram of the present invention as applied to a banknote;and

FIG. 2 is a schematic diagram of system for cleaning and analyzing abanknote in accordance with the present invention; and

FIG. 3 is an enlarged cross-section depicting the result of cleaning abanknote in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to the drawings, there is disclosed a substrate materialthat is formed to include a self-cleaning feature. More specifically,the present invention provides a substrate or coating for a substratethat includes a photo-catalytic material therein when, upon exposure toa source of illumination, reacts in a manner that causes the substratematerial to shed accumulated contaminants and dirt. Further, the presentinvention discloses a system for sorting, cleaning and analyzingbanknotes that are formed using such a substrate material.

In the most general sense, the present invention employs coatings, inksand/or additives that are photo-active. The photo-active nature of thesematerials allows for the self-cleaning of substrates, such as forexample banknotes, by ambient light exposure as well as the cleaning ofprocessed banknotes using equipment with optical excitation, thusincreasing their usable life. The invention is usable with allsubstrates and particularly polymeric substrates such asbiaxially-oriented polypropylene (BOPP).

It is known that semiconductor nanoparticles exhibit variousphoto-catalytic effects. Examples of these include III-V (GaN, GaAs) andII-VI (ZnS, ZnSe, CdS, CdSe, WS₂, MoS₂) semiconductors and various metaloxides such as ZnO, TiO₂, MoO₃, CeO₂, ZrO₂, WO₃, alpha-FeO₃ and SnO₂.TiO₂ (titanium dioxide) is of particular importance for this applicationas it is white in color and inexpensive. Its low cost and availabilityhave been a large factor in its increased use in many areas includingDye Sensitized Solar Cells (DSSC), selective oxidation, disinfection andmetal corrosion prevention

Not all forms of TiO₂ are highly photo-catalytic. The mostphoto-reactive form is the anatase phase which can be obtained by hightemperature conversion. It is of particular note that the anatase formof TiO₂ can be excited by ultraviolet energy having a wavelength in the200 nm-400 nm region and more particularly in the 360 nm-390 nm region.This is advantageous in that light sources such as low cost LED flashlamps, arc sources and lasers are all available for excitation in thisenergy range and are thus anticipated for possible use within the scopeof the present invention. Further, those metal oxides andsemi-conductors listed above are equally applicable and intended to fallwithin the scope of the present invention as being suitablephoto-catalytic materials within the scope of the present invention. Inaddition, modified TiO₂ can be used which has different absorption,interfacial charge transfer and absorption characteristics.

Photo-catalytic degredation with illuminated TiO₂ of a large number ofsubstrates including soot and carbonaceous contaminants is primarilyascribed to the strong oxidation potential of its valence band holes andsurface OH radicals. In some cases, direct electron transfer from theconduction band to O₂ leads to degradation through O⁻ ₂ reactions. Inone embodiment, as shown at FIG. 1, when excitation energy such asambient lighting is directed onto the photo-catalytic materials,electrons and holes can be generated and transferred to O₂ and OHrespectively (1). More particularly, when a stronger effect is desired,ultraviolet energy is directed onto the anatase titanium dioxide, moreelectrons and holes can be generated and transferred. In either case,these charge transfers convert water molecules from the ambient air intohydroxyl radicals (2) that in turn cause chemical oxidation andreduction reactions take place (3). In this specific example, at theanatase titanium dioxide or in general the semiconductor particlesurface. In effect, the hydroxyl radicals attack organic (carbon-based)dirt molecules and break them up into smaller fragments that are mucheasier to remove (4). In this regard, the present invention employsphoto-catalytic semiconductor materials or photo-catalytic metal oxidesas an additive to a banknote either as a coating, ink used for printingof the banknote or within the substrate itself. In a more specificembodiment, the present invention employs anatase titanium dioxide as anadditive to a banknote either as a coating, ink used for printing of thebanknote or as the substrate itself. Since the reactions happen at theanatase titanium dioxide material, preferably positioned on the verysurface of the banknote, they attack the lowest layers of the dirt,loosening encrusted dirt particles very effectively by chipping themaway from the inside out. This is in clear contrast to normal cleaning,where the dirt is scrubbed from the outside in.

The invention further discloses a system at FIG. 2 for evaluating thestatus and fitness of a banknote in order to determine whether it shouldremain in circulation. The system may be a standalone system orincorporated onto banknote counters and sorters. As the banknotes passthrough the system, the system employs a sensor to read various fitnessdata from the banknote. Based on the date the system makes a decision toeither pass or clean the banknote. When the banknote is routed to thecleaning process, a cleaning module applies energization light for toinitiate the photo-catalytic process and removes the photo-catalyticreaction products using a wash, gas jet, or mechanical means. Once thebanknote is cleaned, it is re-evaluated using a fitness sensor thateither returns the note to circulation or sends the banknote to adestroyer device or reject sorter bin.

It is therefore an object of the present invention to provide asubstrate material that provides high security against fraud whileexhibiting increased durability and self-cleaning characteristics. It isa further object of the present invention to provide a substrate thatthrough the use of coatings, inks and additives that are photo-activeand catalytic to reactions which are effective in breaking up organiccontaminants or dirt to allow for the self-cleaning of banknotes byambient light exposure as well as the cleaning of processed banknotesusing equipment with optical excitation, thus increasing their usablelife. It is still a further object of the present invention to provide asystem that using a certain class of fitness parameters causesidentified banknotes to be redirected to a cleaning module prior toreevaluation and then a determination that they are either returned tocirculation or destroyed.

While there is shown and described herein certain specific structureembodying the invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts may be madewithout departing from the spirit and scope of the underlying inventiveconcept and that the same is not limited to the particular forms hereinshown and described except insofar as indicated by the scope of theappended claims.

What is claimed:
 1. A substrate material comprising: a base sheetmaterial; and a photo-catalytic material on or about said base sheetmaterial; wherein said photo-catalytic material, when energized,generates the transfer of electrons and holes that generate hydroxyl andother radicals that in turn cause chemical oxidation and reductionreactions on or about said sheet material.
 2. The substrate material ofclaim 1, wherein said substrate is a banknote.
 3. The substrate materialof claim 1, wherein said photo-catalytic material is selected from thegroup consisting of: an additive within said base sheet material, acoating on the surface of said base sheet material and an additivewithin ink used to print on the surface of said base sheet material. 4.The substrate material of claim 1, wherein said photo-catalytic materialis a nanoparticle metal oxide material. The substrate material of claim1, wherein said photo-catalytic material is modified TiO₂.
 5. Thesubstrate material of claim 1, wherein said photo-catalytic material isa nanoparticle semiconductor material.
 6. The substrate material ofclaim 1, wherein said photo-catalytic material is anatase TiO2.
 7. Thesubstrate material of claim 6, wherein said anatase TiO2 is innanoparticle form.
 8. The substrate material of claim 1, wherein saidphoto-catalysis is initiated using ultraviolet energy having awavelength in the 200 nm-400 nm region.
 9. The substrate material ofclaim 1, wherein said photo-catalysis is initiated using ultravioletenergy having a wavelength in the 360 nm-400 nm region.
 10. Thesubstrate material of claim 1, wherein said oxidation and reductionreactions effect a cleaning of the surface of the substrate material.11. The substrate material of claim 10, further comprising: a systemthat employs a sensor to determine fitness data relating to saidsubstrate; said system making a decision based on said fitness data toclean the substrate said system applies energy to initiatephoto-catalysis; removes dirt from said substrate using a wash, gas jet,or mechanical means; and reevaluates the fitness of said substrate usinga sensor to determine whether to return the substrate to circulation orto destroy or reject the substrate.
 12. A system for cleaning asubstrate material comprising: a sensor to determine fitness datarelating to said substrate having a photocatalytic material on or aboutsaid substrate; making a decision based on said fitness data to cleanthe substrate; applying energy to excite said photo-catalytic materialand initiate photo-catalysis; removing dirt from said substrate using awash, gas jet, or mechanical means; and reevaluating the fitness of saidsubstrate using a sensor to determine whether to return the substrate tocirculation or to reject or destroy the substrate.
 13. The system ofclaim 12, wherein said substrate is a banknote.
 14. The system of claim12, wherein said photo-catalytic material is selected from the groupconsisting of: an additive within said base sheet material, a coating onthe surface of said base sheet material and an additive within ink usedto print on the surface of said base sheet material.
 15. The system ofclaim 12, wherein said photo-catalytic material is a nanoparticle metaloxide material.
 16. The system of claim 12, wherein said photo-catalyticmaterial is a nanoparticle semiconductor material.
 17. The system ofclaim 12, wherein said photo-catalytic material is anatase TiO2.
 18. Thesystem of claim 17, wherein said anatase TiO2 is in nanoparticle form.19. The system of claim 12, wherein said photo-catalysis is initiatedusing ultraviolet energy having a wavelength in the 200 nm-400 nmregion.
 20. The system of claim 12, wherein said photo-catalysis isinitiated using ultraviolet energy having a wavelength in the 200 nm-400nm region.